by cytokines, hormones and cell adhesion molecules. In addition, we have new observations suggesting that retinoids can accelerate the process of lymphocyte formation and their effects on the bone marrow merit further study. For example, these widely prescribed drugs hold promise as agents for boosting recovery of the immune system. We are combining special knock-in mice with cell sorting, cell culture and transplantation techniques to systematically investigate these and other questions. Our lab discovered that Toll-like receptors (TLR) are expressed on hematopoietic cells, allowing these cells to recognize bacterial/viral products and several important outcomes of this interaction have already been identified. For example, TLR ligands stimulate stem cells to enter cycle and begin differentiating, while myeloid restricted cells complete their maturation. The ligands arrest B cell production and cause lymphocyte progenitors to generate several types of dendritic cells. While these new mechanisms may have survival value, we believe there are also circumstances where stem and progenitor cells need to be protected from such substances. Our lab is using defined culture conditions to study mechanisms associated with an apparent re-programming of lymphoid progenitors. In addition, we are tracking the same phenomena in HSV infected mice. Project Description Page 6